1. Field of the Invention
The present invention relates to thin films that can be used as a sensing layer in a range of sensing devices, applications thereof and a method of manufacturing. More in particular the present invention relates to low-power sensing, for example sensing of chemical and biochemical analytes in a gas phase or liquid phase.
2. Description of the Related Technology
Low-power chemical sensing is an enabler for emerging wireless autonomous transducer applications. These applications include energy scavengers that ensure autonomous operation over extended periods of time. Therefore, low-power circuits are needed because the power generated by energy harvesting from the environment is limited. Besides low-power RF-communication, digital processing and AD-conversion, these applications also include low-power sensors and actuators to interact with the environment.
In addition to the requirement of low-power operation and low operating voltage, the sensors should be sensitive and selective as needed by the targeted applications.
Detection of a specific analyte is often achieved by monitoring a change in the physical properties, for example impedance (conductance, capacitance), dielectric constant, and/or work function of a sensing material. Many different materials are used for sensing, ranging from polymers to inorganic materials such as oxides, nitrides, and semiconductors.
For example, for sensitive and selective detection of gases, metal oxides including semiconducting and insulating materials are often used as gas-sensitive layers in gas sensing devices. These devices need to be heated to well above 100° C., typically up to 400° C. in order to enhance gas adsorption and desorption. Operating the sensing devices at elevated temperatures is also required to enhance gas diffusion into the metal oxide layer. Such is a disadvantage e.g. in terms of power consumption and durability of the material.
In U.S. Pat.No. 2005/0235735 a gas sensor is described wherein the adsorption probability of a gas is controlled by means of an electric field. Such gas sensors can therefore be made subject to electrical modulation of their sensitivity to various gases. An improved selectivity for a target gas may be obtained, and operating temperatures can be reduced to below 200° C. These devices also need to be heated for proper operation.
However, the power consumption for heating is excessive compared to the power budget dictated by wireless, autonomous transducer systems. Thus for low-power applications, there is a need for sensitive and selective sensors that can operate at room temperature or ambient temperature.
Metal oxide nanowires have also been used for gas sensing. The sensitivity of nanowire devices can be high because of their high surface-to-volume ratio. An electric field can influence the surface properties and molecular adsorption in the case of nanowire devices. Certain chemical reactions that have high activation energy can be enhanced or even reversed with an electric field at room temperature or ambient temperature, which is a very important property for low-power gas sensors. The electric field enables both enhancement of gas adsorption (sensing) and desorption (refreshing) in the device by modulating the surface states of the sensing materials. This implies that the electric field can be a substitute for temperature both in sensing and refreshing the nanowire sensors. Moreover, the adsorption of a particular kind of molecules is characterized by a certain activation energy, which can be reduced by application of an electric field, improving the selectivity of the device.
For example, it has been reported by Z. Fan et al in “Gate-refreshable nanowire chemical sensors”, Appl. Phys. Lett. 86, 123510 (2005), that devices comprising 60 nm ZnO nanowires on top of a 500 nm thick gate oxide layer require a gate voltage of 60 V to refresh after exposure to 10 ppm NO2. In this case, the required electric field is approximately 1.2 MV·cm−1. Lower voltages that are within the requirements of autonomous systems can be achieved by using thinner gate oxides.
However, the integration of nanowires in a semiconductor fabrication process is complicated if not impossible due to thermal constraints during the growth of nanowires and due to problems related to the positioning of individual nanowires on a chip.